NEW DEVELOPMENT OF A MICROINDENTATION SYSTEM TO DETERMINE MECHANICAL PROPERTIES OF POLYMERS E. Ramakers-van
1,2 Dorp ,
T.
1 Haenel ,
F.
3 Sturm ,
B.
2 HausnerovΓ‘
and B.
1 Moeginger
PO-48 Objectives: Indentation techniques such as static hardness testing on macro- micro- or nanoscale as well as dynamic nanoscale indentation for viscoelastic properties are available. However, a microscale dynamic indentation method is not available on the market today. The aim of this study is to present a quasi-static and a dynamic microindentation method to determine viscoelastic properties with a conventional Dynamical Mechanical Analysis (DMA). Materials and Methods:
Results and discussion:
Samples made of Polycarbonate (PC), Polybutylene terephthalate (PBT), annealed / unannealed Polypropylene (PP) and Thermoplastic polyurethane (TPU) were investigated by microindentation with respect to their local viscoelastic properties using a tungsten needle indenter and 3 diamond indenters with Berkovich, Vickers and Rockwell geometry. The indenters were implemented in a DMA 242 C (Netzsch, Germany) with special indenter holders.
The quasi-static force displacement curves of all tested polymers are shown in Fig 2. The four polymers can clearly be distinguished.
β
π¬ = A Fs* As*
π
π π¨
β
β ππ π¨βπ
(1)
contact area of indenter [Β΅m] measured complex load [F] measured complex amplitude [Β΅m]
A comparison of the measured complex moduli of PC, PBT and PP agree well with the literature values (tensile tests) [5], Fig 3.
Fig 2: Quasi-static microindentation with tungsten needle of different polymers
Fig 1: Special indenter holder for DMA
The measurements with the tungsten needle indenter were performed quasistaticly to determine the βstaticβ stiffness (S) at room temperature with frequency 1Hz. The load was increased in 30 steps from 0.05 N up to 2.0 N. The measurements with the other indenters were performed using dynamic micro-indentation in the DMA penetration mode to determine the complex modulus E* at room temperature, with frequency 1 Hz. The results were compared to the corresponding complex moduli of all polymers determined by three point bending at room temperature, the frequency 1 Hz, maximum load of 5 N and a maximum amplitude of 25 Β΅m. Furthermore, values from literature were taken to complete the comparison.
At the beginning, all force displacement curves are superimposed. Due to the small contact area of the needle, only the internal stiffness of the system is measured. Evaluation starts above 0.5N. TPU possesses the lowest static stiffness, whereas PBT and PP lay in between PC and TPU. This represents the expected differences in mechanical properties of the tested polymers. Furthermore, evaluation of static stiffness of annealed (S= 4.7 Β± 0.2 [N/mm]) and unannealed (S= 3.9 Β± 0.1 [N/mm]) PP samples show that annealing effects can be identified both locally and globally. If the geometry factors of the different diamond indenters are known [1], Table 1, equation (1) representing the Oliver and Pharr approach [2, 3, 4] can be used to calculate the complex moduli. Table1: Geometry factors of different diamond indenters [1]
indenter Berkovich
Acknowledgements: This study was supported by the German Ministry of Education and Research. Grant No.: 03FH051PX4. The author also thanks the Graduates Institute, Bonn-Rhein-Sieg University of Applied Sciences for supporting this work by granting a scholarship.
Vickers Rockwell
References: [1] [2] [3] [4] [5]
A. C. Fischer-Cripps, Nanoindentation, 2002 W. C. Oliver, G. M. Pharr, J. Mater. Res. 7, 1565 (1992) G. M. Pharr, W. C. Oliver, F. R. Brontzen J. Mater. Res. 7, 613 (1992) A. C. Fischer-Cripps J. Mater. Res. 19, 2981 (2016) Datasheets of tested polymers
geometry factor π΄ = 24.49 β ππΏ2 [ππ2 ] π = 65.27Β° 2
2
2
2
π΄ = 24.504 β ππΏ [ππ ] π = 68Β°
π΄ = 20.308 β ππΏ [ππ ] π
= 200Β΅π (πππππ’π π‘ππ)
Affiliations
Fig 3: Comparison of dynamic microindentation with literature values and 3-point bending DMA
Microindentation depends on differences in local mechanical properties which dissolve in the bulk for e.g. tensile test or 3 point bending. This can be especially seen for TPU where literature value from tensile testing is more than ten times larger than the measured complex moduli by microindentation. Conclusions: It is shown that different polymers and small inhomogeneous mechanical properties distribution can be distinguished by quasi-static microindentation. Furthermore, the measured complex moduli from the dynamic microindentation with different diamond indenters were all in good agreement with the 3-point bending moduli and the literature values. In summary, this study showed that it is possible to modify a DMA to perform both quasi-static and dynamic micro-indentation to determine local mechanical properties of polymers on a microscale scale. Contact e-mail:
[email protected] 1. Department of Natural Science, Bonn-Rhein-Sieg University of Applied Sciences, Germany 2. Faculty of Technology, Centre of Polymer Systems, Tomas Bata University in ZlΓn, Czech Republic 3. SGS Institut Fresenius GmbH, Taunstein, Germany
